1. Field of the Disclosure
The present disclosure relates generally to a stopper for a syringe assembly. More particularly, the present disclosure relates to a thermoplastic elastomer (TPE) stopper that meets the desired material properties of a stopper for a syringe assembly.
2. Description of the Related Art
Syringe assemblies are well known in the medical field for dispensing fluids, such as medications. A conventional syringe typically includes a syringe barrel with an opening at one end and a plunger mechanism disposed through the opposite end. The plunger mechanism typically includes a plunger rod extending through the barrel, with a plunger head or stopper disposed at the end of the plunger rod within the syringe barrel, and with a finger flange at the other end of the plunger rod extending out of the syringe barrel. In use, the plunger rod is retracted through the syringe barrel to aspirate or fill the syringe barrel with a fluid, such as a medication, with the plunger rod extending out from the rear end of the syringe barrel. For delivery of the medication to a patient, the opening of the syringe barrel is adapted for fluid communication with a patient, such as through a hypodermic needle fitted at the front end of the syringe barrel or through a luer-type fitting extending from the syringe barrel for attachment with a fluid line of a patient. Upon the user applying a force to depress the plunger rod and stopper through the syringe barrel towards the front end of the syringe barrel, the contents of the syringe are thereby forced out of the syringe barrel through the opening at the front end for delivery to the patient. Such an operation is well known in the medical field, and medical practitioners have become well accustomed to the use of such common fluid delivery procedures through standard syringes.
Syringe assemblies require slow and controlled initiation and maintenance of sliding movement of one surface over another surface. It is well known that two stationary surfaces having a sliding relationship often exhibit sufficient resistance to initiation of movement that gradually increased pressure applied to one of the surfaces does not cause movement until a threshold pressure is reached, at which point a sudden sliding separation of the surfaces takes place. This sudden separation of stationary surfaces into a sliding relationship is herein referred to as “breakout”.
A less well known, but important frictional force is “breakloose force”, which refers to the force required to overcome static friction between surfaces of a syringe assembly that has been subjected to sterilization (including autoclaving or other processes) and may have a slight deformation in one or both of the contacting surfaces of the syringe assembly, for example in the syringe barrel. In addition to autoclaving, parking of the assembly can further increase the breakloose force.
Breakout and breakloose forces are particularly troublesome in liquid dispensing devices, such as syringes, used to deliver small, accurately measured quantities of a liquid by smooth incremental line-to-line advancement of one surface over a graduated second surface. The problem is also encountered in devices using stopcocks, such as burets, pipets, addition funnels, and the like where careful dropwise control of flow is desired.
A critical performance requirement of a stopper is achieving high leak pressure, i.e., the ability of a stopper to maintain a leak-free syringe while maintaining low breakloose and sustaining forces.
The problems of excessive breakout and breakloose forces are related to friction. Friction is generally defined as the resisting force that arises when a surface of one substance slides, or tends to slide, over an adjoining surface of itself or another substance. Between surfaces of solids in contact, there may be two kinds of friction: (1) the resistance opposing the force required to start to move one surface over another, conventionally known as static friction, and (2) the resistance opposing the force required to move one surface over another at a variable, fixed, or predetermined speed, conventionally known as kinetic friction.
The force required to overcome static friction and induce breakout is referred to as the “breakout force”, and the force required to maintain steady slide of one surface over another after breakout or breakloose is referred to as the “sustaining force”. Two main factors contribute to static friction and thus to the breakout or breakloose force. The term “stick” as used herein denotes the tendency of two surfaces in stationary contact to develop a degree of adherence to each other. The term “inertia” is conventionally defined as the indisposition to motion which must be overcome to set a mass in motion. In the context of the present invention, inertia is understood to denote that component of the breakout force which does not involve adherence.
Breakout or breakloose forces, in particular the degree of stick, vary according to the composition of the surfaces. In general, materials having elasticity show greater stick than non-elastic materials, particularly when the surfaces are of similar composition. The length of time that surfaces have been in stationary contact with each other also influences breakout and/or breakloose forces. In the syringe art, the term “parking” denotes storage time, shelf time, or the interval between filling and discharge. Parking generally increases breakout or breakloose force, particularly if the syringe has been refrigerated during parking.
A conventional approach to overcoming breakout has been application of a lubricant to a surface-to-surface interface. Such conventional lubricated stoppers have the disadvantage of being soluble in a variety of fluids, such as vehicles commonly used to dispense medicaments. In addition, these lubricants are subject to air oxidation resulting in viscosity changes and objectionable color development. Further, they are particularly likely to migrate from the surface-to-surface interface. Such lubricant migration is generally thought to be responsible for the increase in breakout force with time in parking.
Additional problems with applying a lubricant to a surface of a stopper is that such a lubrication step requires costs in lubricants and lubing instruments, time and energy to operate and perform the lubrication step, and the stopper must be removed from an automated assembly process to be lubricated.
For these reasons, there is a need for a better syringe assembly system to overcome high breakout and breakloose forces whereby smooth transition of two surfaces from stationary contact into sliding contact can be achieved and there is a need for a stopper that exhibits the required performance characteristics and that does not require the additional lubrication step.